1. Field of the Invention
The invention relates to an integrated optical circuit comprising a first device having an output for emitting radiation with a first state of polarization, a second device having an input for receiving radiation with a second, different state of polarization, and a polarization convertor connected between the output and the input.
The polarization convertor converts radiation guided in a TE mode into radiation guided in a TM mode or vice versa. The TE (transverse electric) mode is a mode in which E.sub.y is the dominant electric component and the TM (transverse magnetic) mode is a mode in which H.sub.y is the dominant magnetic component of the electromagnetic field of the radiation. The y-axis is an axis in the plane of the slab-type substrate of the circuit and perpendicular to the direction of propagation of the radiation. The conversion ratio is the ratio of the amount of radiation in one mode which the convertor converts to radiation in the other mode.
2. Discussion of the Related Art
A circuit as described in the opening paragraph is known from U.S. Pat. No. 5,185,828, disclosing an optical input section for a coherent optical receiver operating on the basis of polarization diversity. The first device is a local radiation source, emitting TE polarized radiation. The polarization convertor changes the state of polarization to 50% TE and 50% TM polarized radiation. The second device comprises a mixer for combining the converted local oscillator radiation with signal radiation, and a detection circuit. The polarization convertor comprises a straight waveguide having a geometric structure consisting of a period sequence of waveguide sections with different widths. Drawbacks of the polarization convertor are that its length is relatively large, of the order of millimeters, and that it has a small bandwidth due to its length.